Protective colloid-stabilized polymers are used especially in the form of their aqueous dispersions or water-redispersible polymer powder in many kinds of applications, for example as coating materials or adhesives, for a very wide range of substrates. Protective colloids used are as a rule polyvinyl alcohols. The use of polyvinyl alcohol is worthwhile because, in comparison with systems which are stabilized by low molecular weight compounds (emulsifiers), it itself contributes to the strength (e.g. adhesive strength values in tile adhesive). To date, vinyl esters and ethylene have preferably been used as monomers for the preparation of redispersible powders since stabilization of acrylate copolymers or styrene-acrylate copolymers by polyvinyl alcohol is not easy to accomplish.
The preparation of these dispersions is traditionally effected by emulsion polymerization in the batch process. This procedure is distinguished by high flexibility and is thus preferably to be encountered in plants with a large variety of products. In addition, this procedure is substantially less demanding than a continuous process in terms of process engineering. A disadvantage is the substantially lower cost-efficiency of the batch process in comparison with the continuous process.
The continuous process is thus the correct choice for plants with a high output in combination with a small variety of products. A key problem in the case of the continuous emulsion polymerization is establishing the correct conditions for permanent nucleation of new latex particles. The favoured plant configuration is the stored tank cascade because such reactors are already used in batch operation and would thus be capable of being used with only little economic conversion effort for the continuous process. Usually, more or less pronounced variation of the product properties and of the process parameters is encountered, which in the extreme case may lead to unusable products or damage to the plant.
U.S. Pat. No. 2,703,794 describes a process for the continuous emulsion polymerization of vinyl acetate and ethylene in the presence of an emulsifier and optionally water-miscible organic solvents, water, emulsifier, monomers and initiator being metered in simultaneously and continuously. The process is carried out without seed latex and in a single pressure-resistant reactor. Pronounced deposits on the reactor wall and varying particle sizes are disadvantageous.
U.S. Pat. No. 3,551,396 describes a process for the preparation of vinyl acetate homopolymer dispersions in a loop reactor, in which monomer and water are continuously circulated, additional monomer and initiator being metered in continuously and polymer formed being removed continuously from the reactor. The polymerization is carried out without seed latex and in a single pressure-resistant reactor.
DE-A 2555142 describes a continuous process for the preparation of polymers of ethylenically unsaturated monomers by the emulsion polymerization process in a plurality of polymerization reactors connected in series. For this purpose, a first reactor is charged with a preemulsion comprising water, initiator, a part of the monomers and further auxiliaries, the dispersion obtained is transferred to a second reaction vessel, the residual proportion of the monomers is then fed in and finally, in a last reaction vessel, the dispersion is completely polymerized by addition of further initiator. At the end of the reactor cascade, a heat exchanger is arranged for cooling the polymer dispersion obtained.
DE-A 2456576 describes a plant configuration consisting of a coolable stirred autoclave for the preparation of the preemulsion comprising monomer, emulsifier, reducing agent and water, a reactor for the main polymerization and a tube for the postpolymerization. The polymerization is carried out in the presence of a redox initiator system, the reducing agent being used in a large excess and the oxidizing agent being metered into the reactor. The monomers are metered in so that the concentration thereof does not exceed a defined limit. The rapid formation of deposits on the wall which leads to low plant availability is disadvantageous in this process. Inter alia, the strongly varying properties of the products are the cause of this.
U.S. Pat. No. 4,164,489 describes a process for the continuous preparation of vinyl acetate-ethylene dispersions, which is distinguished by use of a seed latex which is likewise fed in continuously. A disadvantage is that this seed is an independent product which has to be prepared additionally for this purpose (small particle size, about 200 nm), undesired constituents of the formulation, such as emulsifiers, being entrained into the product.
EP-A 1174445 and EP-A 1323752 describe processes for the continuous preparation of vinyl acetate-ethylene dispersions in at least two pressure-resistant reactors connected in series, the polymerization being initiated with a redox initiator system and the reduction component being fed at least partly to the first pressure-resistant reactor.
Petrocelli et. al. (Polym. Sci. Eng., 80, 544) likewise describe a process using seed latex. In this case too, a finely divided (particle size about 200 nm), emulsifier-stabilized dispersion is used. The basic idea in this seed latex process is the introduction of new latex particles having a small size, which then grow to the desired particle size in the course of the polymerization. Internal new nucleation is to be suppressed by the seed latex.
As a further alternative for increasing the productivity, the literature describes the combination of heat exchangers with batch reactors. In these cases, the batch reactor and the heat exchanger are connected to form a circulation in such a way that the medium to be cooled is fed from the reactor into the heat exchanger and is passed back again from there into the reactor. The additional cooling capacity of the external heat exchanger can be used for reducing the polymerization time. That large amounts of substance have to be transported at high speed through the heat exchanger in order to keep the process efficient is a disadvantage of this principle. The pumps required for this transport process are high-performance pumps with a corresponding high energy and shear load for the medium to be transported.
EP-A 834518 describes a batchwise polymerization process in which the polymerization takes place in a reactor having an external cooling circulation, in which it is essential to use a low-shear pump in order to avoid impairing the products.
WO-A 03/006510 describes a batchwise polymerization process in a polymerization reactor having an external cooling circulation, in which the polymerization batch is circulated by means of a low-shear cylinder or tube diaphragm pump in order to avoid impairing the products by shearing.
WO-A 02/59158 describes a polymerization process in a reactor having an external cooling circulation, in which the polymerization batch is circulated by means of a pump and at least a portion of the monomer has to be metered into the external circulation.
A disadvantage in the case of the batchwise polymerization process in which large-volume batches comprising dispersed polymer particles have to be circulated by pumping is the susceptibility thereof to coagulation owing to the shear forces introduced by the pump.
It was an object of the present invention to develop an economical and technologically improved process for the preparation of vinyl ester-ethylene copolymer dispersions which avoids the variations in the continuous process management and high mechanical loading of the processes with external cooling. It was necessary to take into account problems which can occur not only in the preparation but also during the use of the desired products. Particularly with the use of polymers in the form of their redispersion powders for improving the mortar properties, a main field of use of redispersion powders, the formulations must remain stable over a certain time and must not substantially change their processing consistency (viscosity stability or cement stability). In the concrete and mortar industry, moreover, mechanical properties, such as the compressive strength, the porosity and hence the air void content, play an important role. The hydraulically setting systems treated with dispersion powder should moreover provide even better adhesion compared with the untreated systems. All these product properties must still be guaranteed on changing over from the traditional batch process to the more economical continuous process.